Upper mantle anisotropy beneath Indochina block and adjacent regions from shear-wave splitting analysis of Vietnam broadband seismograph array data

The Indochina block is located at the junction of four major plates. It has a close relationship with the uplift of Tibet plateau and the sea-floor spreading of the South China Sea in the geological evolution histories. We deployed a temporary broadband seismograph array in Vietnam from February 2000 to October 2005, and measured shear-wave splitting of core-refracted phases to investigate the upper mantle anisotropy beneath the Indochina block and adjacent regions. The observed delay times of up to 1.5 s are large, indicating a mantle source for the anisotropy. Two distinct regimes of seismic anisotropy are revealed. The fast polarization directions in the northern Indochina block with a main trend of N100°E are almost consistent with the direction of both the surface displacement and the absolute plate motion. These observations suggest the anisotropy is possibly generated by the present-day asthenospheric flow caused by the India-Eurasia collision. The fast polarization directions in the southern Indochina block and the Hainan island are nearly N60°E. The anisotropy in this area may reflect a combination of the India-Eurasia collision and the past orogenies in the lithosphere, or an abrupt variation in the asthenospheric flow.